Servo motors are used in a wide range of industrial and military applications to position equipment rapidly and with a high degree of positional accuracy. Examples of these uses include industrial robots which are used in manufacturing operations, and military vehicle artillery units. Servo electronic equipment is generally comprised of a three-phase servo amplifier and a three-phase servo motor that are electrically connected by a three-phase servo motor power cable, along with the associated control system.
Conventional three-phase servo motor power cable designs exhibit high values of parasitic electrical capacitance between the power conductors and the shield/ground of the cable. This parasitic conductor to shield/ground capacitance forces the designer to utilize electromagnetic interference (EMI) control techniques that severely impact other important equipment parameters including size, weight, power, and cooling requirements. Servo system design constraints are particularly stringent in a military environment, such as onboard a ship or field artillery vehicle, where stringent military standards for design and performance specifications must be met.
Existing three-phase servo motor power cables of the prior art do not meet the stringent design criteria that exist for some modern servo electronic equipment configurations. For example, a typical cable design of the prior art utilizes three motor conductors with three ground wires which are in close proximity to the three power conductors. The six conductors are covered with an overall shield and insulating jacket. The close proximity of the power conductors to the ground wires and the shield result in a large parasitic conductor to shield/ground capacitance, thereby not complying with the system design criteria specified above.
In an effort to reduce the detrimental effects of parasitic conductor to shield/ground capacitance, designers of servo electronic equipment have devised a solution of inserting common mode inductors in the motor power circuit. While this technique reduces the current flowing in the parasitic capacitance by creating a radio frequency (RF) tuned circuit, it does not reduce the actual parasitic conductor to shield/ground capacitance. This technique has limited effectiveness, and it is typically limited to applications where the circuit can be tuned for a particular installation. Also, because the inductors are large, the added space, weight, and power dissipation requirements are not tolerable in a military application.
The value of parasitic conductor to shield/ground capacitance in a particular cable design is dependent on the dielectric material that is used in the cable. Air as a dielectric has a relative permittivity (εr) of 1.0. In comparison, the relative permittivity of the dielectric material used as electrical insulation in conventional cables is 2.0 or higher. Because electrical capacitance is directly proportional to the relative permittivity of the dielectric, the parasitic conductor to shield/ground capacitance of a power cable can be reduced by using air as a dielectric. For example, Polk, U.S. Pat. No. 2,752,577, disclosed a coaxial cable for use in the transmission of radio frequency (RF) power that uses air or another gas as a dielectric material. Unlike the coaxial three-phase servo motor power cable of the present invention, Polk disclosed a single inner electrical conductor, and the single inner conductor was supported by dielectric beads. Therefore, Polk is not useful in a three phase servo motor power cable design.
The high values of parasitic conductor to shield/ground capacitance seen in coaxial three-phase servo motor power cables of the prior art results in a corresponding parasitic electrical current flow, directly contributing to the several issues previously discussed. The additional detrimental effects of this parasitic current flow includes saturation of the circuitry's ground fault detector current transducer, rendering it useless; interfering with sensitive circuits such as the motor resolver and motor RTD; and producing an electrical shock hazard to personnel from induced voltages.
Therefore, there is a current need by servo electronic equipment designers for a three-phase servo motor power cable that meets servo motor power and performance requirements by significantly reducing parasitic conductor to shield/ground capacitance values, while also meeting strict design requirements for electromagnetic interference, radiated and conducted emissions, and personnel electrical safety. Accordingly, satisfying the need for this three-phase servo motor power cable will enable servo electronic equipment designers to utilize servo motor systems that are smaller, lighter in weight, dissipate less power, and therefore require less cooling than existing system designs while also meeting the strict design requirements.